Planning my 750W Off-Grid System - Am I doing it right?

Tobit

I just drew up a rough schematic for a 750W off-grid system for my shed on some remote land in the Missouri Ozarks I own and will be moving to soon. My cabin will have no power, the limited devices requiring power will all be in the shed. I am on a southern slope at ~1400' with a yearly insolation average of 4.5. My largest power consumer will be my 5.2 cu. ft. freezer (0.85A current when running, ~11.6A inrush current when starting).

I will let the drawing explain most of it but here are a few details:

Panels will be ground mounted with the shed tucked in behind some trees to the north of the array. I already have all the panels, the two 100W pnales were from some early experiments and the 275W panels were purchased because of some excellent pricing, $1.22/watt delivered, so it was hard to refuse at the time. No other hardware has been purchased yet. Battery bank will be 4x6V 225AH type to be determined.

At the panels, I am planning for a MidNite MNPV6 combiner with the following installed in it: 1x 10A PV breaker, 1x 8A PV breaker, 1x MNDC-GFP63 breaker, and 1x SPD-300 surge protection device. The panels and the ground bus in the MNPV6 will be bonded to a single ground rod.

In the shed will be a breaker box with 3x30A breakers; one between the PV array and the KID, one battery breaker, one DC load breaker and battery negative bus. A ground rod will be driven just outside of the shed for hardware in the shed such as the kid.

Am I on the right path? I haven't made accommodations to this drawing for the inverter for my freezer yet as I still have a few things to sort out. Two questions that do come to mind; one, have I sized the breakers in the MNPV6 properly and should I install an additional SPD-300 inside the shed breaker box? Sorry for not drawing the GFP63 better but hopefully you will get the idea.

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AuricTech

Re: Planning my 750W Off-Grid System - Am I doing it right?

I take it you plan to run the 2 100W panels in series?

Cariboocoot

Re: Planning my 750W Off-Grid System - Am I doing it right?

I hate to say this but you're going about it all wrong. The right way to size an off-grid system is by the Watt hours it can supply.

The problem is that you are trying to use panels of different specifications which probably will not work well together.

Your 275 Watt panels have a Vmp of 31. Your 100 Watt panels in series have a Vmp of (2 * 17.8 ) 35.6. That's more than 3 Volts difference. That will mean a power loss for sure. In practice it is hard to predict exactly how they would function together, but think of the 275 Watt panels being 'pulled up' above Vmp resulting in a loss of current output (and thus power) whereas the 100 Watt panels would be 'pulled down' in Voltage also resulting in a drop in power.

Your panel string fuses are sized wrong. I would bet the spec for the 275's with their 9.5 Isc would be 15 Amps and the 100's with their 5.9 Isc might be 10 or even 15 as well. If you use 10 and 8 they will pop repeatedly.

If they work together you have an array of 750 Watts. In theory on a 24 Volt system that should manage 24 Amps peak current which is fine for a single string of 225 Amp hour batteries. The issue is how much the different Vmp's will detract from the power output. If it's 14% off (and it could be) you'll get 20 Amps which would still work. If it's more than that then you're getting into the area where loads could tip the balance in the wrong direction.

AuricTech

Re: Planning my 750W Off-Grid System - Am I doing it right?

Have you considered using the 2 100W panels for a small PV system for the cabin, just enough for a few 12VDC lights?

ETA: Of course, if you did that, you would want to get another 275W panel for the main PV system in the shed.

Cariboocoot

Re: Planning my 750W Off-Grid System - Am I doing it right?

Oh forgot to mention that the 31 Vmp may not be enough to charge a 24 Volt system on their own due to V drop from heat and wiring losses. A 24 Volt system usually has an array Vmp of 35 or more to make up for this.

Tobit

Re: Planning my 750W Off-Grid System - Am I doing it right?

AuricTech wrote: »

Have you considered using the 2 100W panels for a small PV system for the cabin, just enough for a few 12VDC lights?

ETA: Of course, if you did that, you would want to get another 275W panel for the main PV system in the shed.

Actually yes, I have. Based on what coot has said, that would probably be best.

Unfortunately, I can't use the myriad of calculators out there to say I need an XXX Watt Hour system. Because I honestly don't know. Moving from the city, to a much simpler life in the bush, I have no clue how many hours/day the RV pump I will use to take a shower will run, maybe I won't take a shower every day? The only device I have with some data is my small chest freezer which will be my most important device and have the highest demand. However, I still don't know how long the compressor will run for each day and it will definitely run longer in the summer than the winter.

Cariboocoot

Re: Planning my 750W Off-Grid System - Am I doing it right?

In that case the key question is: what happens if you run out of power?
If the answer is "I start the generator" you're good.
If the answer is "I can wait 'til the batteries recharge from solar" you're good.
If the answer is "There goes the frozen good, my life-saving medicine, and CPAP" you're in trouble.

Tobit

Re: Planning my 750W Off-Grid System - Am I doing it right?

Cariboocoot wrote: »

In that case the key question is: what happens if you run out of power?
If the answer is "I start the generator" you're good.
If the answer is "I can wait 'til the batteries recharge from solar" you're good.

I'm good. I do want to limit generator usage but know there will be times when it will be necessary. Part of moving to the bush for me is cutting back on electrical devices to the bare minimum.

Here is mostly what I will be running in the shed:

Freezer (currently drawing ~84W in my garage according to a Kill-A-Watt)
Shed Lighting (unknown usage)
24V USB chargers for phone and tablet (unknown usage)
120VAC Battery chargers for 18V power tools and rechargeable AAA/AA batteries (unknown usage)
24VDC water pump (haven't picked one yet, no specs/usage known)

Sometime down the road, I may add a satellite internet router but I may cut daily ties to the internet as well.

BB.

Re: Planning my 750W Off-Grid System - Am I doing it right?

The refrigerator/freezer and the Satellite system will probably be your major loads.

And when you use the Kill-a-Watt type meter--There are two major numbers your need:

Watts == Basically the "rate of power" being used (a 100 Watt lamp).

Watt*Hours == The amount of power used (100 Watt lamp * 8 hours of use = 800 Watt*Hours of energy)

It is Watt*Hours that we mostly use to figure out the size of your system.

The Watts are just need to size the wiring and AC inverter (size of wire/inverter/fuses/etc.) to carry the power.

-Bill

Tobit

Re: Planning my 750W Off-Grid System - Am I doing it right?

BB. wrote: »

Watt*Hours == The amount of power used (100 Watt lamp * 8 hours of use = 800 Watt*Hours of energy)

It is Watt*Hours that we mostly use to figure out the size of your system.

Right, but I can't really provide this accurately. I have no clue how many hours a day the freezer will run when I get it to my land 1200 miles away. It will also run less in the winter and more in the summer. One of the reasons I bought the freezer ahead of time was to do some estimating with it in my garage once summer arrives. However, it really hasn't been warm enough here yet. When I first received the unit, day time temps were in the 60's (F) and it used 1.2 KWH during the first 60 hours I had it. It has been off now for the past 12 hours so I can check inrush current later this morning.

I will have no fridge. My space requirements are too minimal to own a fridge, I'd much rather have the larger, stand-alone, chest freezer. I will be building an old-fashioned style ice-box for the few things that I will need to keep cold. I also don't have satellite internet yet and I may never have.

My lifestyle will be changing drastically once I move off-grid, I can't really compare what I use now to what I will use in the future.

Tobit

Re: Planning my 750W Off-Grid System - Am I doing it right?

I guess my biggest mistake was ignorantly thinking I could use a combiner to utilize a 550W series string and a 200W series string into a single array. I could swear I have seen people using a mixture of different specd panels in a single array. Although, I guess just because I see it done doesn't mean it's correct or efficient.

Since I haven't bought anything else yet, the best thing would probably be to add one more 275W panel to my plans in order get the current up to a more proper level to charge a 24V bank. Forget about trying to integrate the 100W panels and use them on something else. 3x275W in series would give me ~93V at 28.6A.

Photowhit

Re: Planning my 750W Off-Grid System - Am I doing it right?

Tobit wrote: »

...3x275W in series would give me ~93V at 28.6A.

No, it will give you 93V at @9amps, The charge controller will convert it into charging voltage and current for your battery bank. The 'Kid' can handle this, but will be a bit more stressed (run hotter) than if you were running at closer to double your system voltage.

Cariboocoot

Re: Planning my 750W Off-Grid System - Am I doing it right?

Tobit wrote: »

I guess my biggest mistake was ignorantly thinking I could use a combiner to utilize a 550W series string and a 200W series string into a single array. I could swear I have seen people using a mixture of different specd panels in a single array. Although, I guess just because I see it done doesn't mean it's correct or efficient.

Since I haven't bought anything else yet, the best thing would probably be to add one more 275W panel to my plans in order get the current up to a more proper level to charge a 24V bank. Forget about trying to integrate the 100W panels and use them on something else. 3x275W in series would give me ~93V at 28.6A.

There are many panels with different specs which can be used together because they "add up right". Unfortunately yours don't. So they would be in the realm where they can be used together, but not very well.

For one thing the 31 Vmp panels will only contribute up to a point; once the battery Voltage has equaled the panel output (with losses) they won't be able to raise Voltage further; the final charging would fall on the 100 Watt panels and they may not be sufficient to do it in a reasonable amount of time.

See what Photowhit said about three 275 Watt panels in series.

Tobit

Re: Planning my 750W Off-Grid System - Am I doing it right?

Photowhit wrote: »

No, it will give you 93V at @9amps, The charge controller will convert it into charging voltage and current for your battery bank. The 'Kid' can handle this, but will be a bit more stressed (run hotter) than if you were running at closer to double your system voltage.

Yeah, the numbers I provided came from the KID calculator.. sorry for not stating it properly. Here's the screenshot from the calc, I realize the kid will be close to maxed out if I go with 3x275 in series.

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Tobit

Re: Planning my 750W Off-Grid System - Am I doing it right?

So where should I be heading with my planning? Again, other than my freezer, I honestly have no clue how often my little chargers, lights, and DC water pump will be used. I know I want a 24VDC battery bank though.

I bought the 2x275W Solarworld panels, admittedly probably prematurely, because I got an amazing deal on the freight at only $25 for both from a NJ warehouse compared to most of the other distributors on the west coast quoting me upwards of $300 just for freight.

I can add another 275W SW panel but with the Kid running close to maxed out, should I forget about the Kid and go with something like a Morningstar TS-MPPT-45 for a bit more buffer or think even further ahead and go with a MN Classic 150? I'd prefer the Kid, long term I like where MN is going with the product and it'd be easy to plug in another Kid later.

Meanwhile, I hooked up a more accurate meter to my freezer this morning. It may be even more efficient than I thought as the Fluke (+/- 1% accuracy) is showing only 0.6A draw compared to my less accurate meter and Kill-A-Watt showing ~0.84A. However, although the Fluke is more accurate, resolution isn't ideal.

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Cariboocoot

Re: Planning my 750W Off-Grid System - Am I doing it right?

I can tell you this: 225 Amp hours @ 24 Volts can power a lot of stuff in a day if you use it right.
I've got a bit more capacity and run the 'frige, satellite Internet & phone, water pumps, lights, etc. The trick is to know when you've got 'power to spare' and make use of it. Hence the pumps only get turned on when the batteries are full. Likewise I can use the microwave for a hot lunch if I want. This sort of management can greatly improve the over-all efficiency of the system.

If I get ambitious this year I may wire a meter into the output and run up the Watt hours to see just how much I can squeeze out.

The Kid would not be maxed out on power: about 26 Amps from three 275 Watt panels. But with the Vmp @ 93 and the system @ 24 there would be a loss of efficiency caused by the large Voltage difference.

Alternative: run the two 275's in series on a Kid and put the two 100 Watt panels in series on an inexpensive PWM controller. You'd get about 17 Amps out of the kid (7.5% rate) and 5 more Amps from the 100 Watters: total 23 Amps or just about 10%. You don't need a big charge controller for 5 Amps, but it does need to be 24 Volt. http://www.solar-electric.com/ss-10l.html Cheaper than a third 275 Watt panel.

BB.

Re: Planning my 750W Off-Grid System - Am I doing it right?

The Kill-a-Watts are only ~$25--They are not as accurate as a Fluke meter.

However, they are usually "close enough" for our needs.

Regarding your present situation--Go ahead and long the kWH usage by the freezer with your K-a-W meter. You will get a good idea of winter/summer performance anyway.

Something else I have done--When I needed to simulate hot climates in an office lab... I built a "chamber" out of cardboard or other similar "scrap/cheap materials" (I have used sheet rock to make an oven to heat plastic sheets for molding into shaped windows too).

Just tape together some large appliance boxes into something large enough to hold your freezer and put it in the box. Usually the self heating of the appliance is enough to raise the internal box air temperature to the needed range.

When I needed to automate the temperature control, I even put a couple of computer fans in and a thermal trap to push cool air into the box and a third fan to gently circulate air flow). In your case, you can probably even use a wall or corner for several sides of the enclosure.

May be way more work than it is worth... And you can just estimate the summer usage. Generally, most people will find that winter solar production is where you will have insufficient solar power production anyway.

One thing to add--As the temperature rises, you may see an increase in power (Watt) usage too (more back pressure for the compressor).

-Bill

Tobit

Re: Planning my 750W Off-Grid System - Am I doing it right?

'coot, I like that idea of putting the 100W panels on their own controller and starting with just the 2x275's on the Kid. I could always add one more 275W panel to the Kid later on if needed. Can I still go and plan on disconnecting breakers, GFP, and SPD located at the panels (use the MNPV6 more as a breaker box without the combiner bar)? Should I also add an SPD in the breaker box in the shed?

Tobit

Re: Planning my 750W Off-Grid System - Am I doing it right?

Cariboocoot wrote: »

Your panel string fuses are sized wrong. I would bet the spec for the 275's with their 9.5 Isc would be 15 Amps and the 100's with their 5.9 Isc might be 10 or even 15 as well. If you use 10 and 8 they will pop repeatedly.

On the spec sheet, would I be looking at the "Max. Reverse Current"? If so, Max. Reverse Current is 16A for the 275's. The spec sheet on the 100W panels show this as a "max. series fuse rating" of 15A.

Cariboocoot

Re: Planning my 750W Off-Grid System - Am I doing it right?

Tobit wrote: »

On the spec sheet, would I be looking at the "Max. Reverse Current"? If so, Max. Reverse Current is 16A for the 275's. The spec sheet on the 100W panels show this as a "max. series fuse rating" of 15A.

Maximum series fuse rating is what you need to protect the panels with.
But here's some more good news: each panel type will be on its own controller and they will be in series. Since that means no parallel panel connections no panel fuses/breakers are needed. One controller, one string of panels. You may want a disconnect on each, but it is not vital: cover panels, output drops to zero, wires can be safely disconnected. You won't have to be doing that very often.

Controller output must still have over-current protection, sized according to the controller specs. Here and on the inverter is where you may find circuit breakers preferable to fuses: circuit protection and disconnect all-in-one. And always remember to connect battery first, then panels on controller (disconnect in reverse; panels first, then batteries). Not all controllers require this but if you follow it it works for every controller.

SPD (when used) should be on the input to the charge controllers. This means you'd need two because of having two controllers. Exact placement depends on length of wire runs and grounding placement. For most installs they will work fine inside at the controller(s) and connected to the main grounding rod. (Sometimes you put them outside on the combiner box and sometimes both locations.)

Personally I dislike GFCI and would only use it on AC outlets in wet areas, not the whole AC line and not on the DC side at all. NEC does not agree with that.

Tobit

Re: Planning my 750W Off-Grid System - Am I doing it right?

Cariboocoot wrote: »

Personally I dislike GFCI and would only use it on AC outlets in wet areas, not the whole AC line and not on the DC side at all. NEC does not agree with that.

Yeah, I was looking at it from the NEC code perspective allowing one a 12" deep trench instead of 18" when DC-GFP is installed on the PV end. However, my land is located in an area where I am not bound to code or inspections so I am probably over-thinking this as I do with many things.

Tobit

Re: Planning my 750W Off-Grid System - Am I doing it right?

On to batteries.

Can I start with the Duracell GC2 AGM's you can get at Sam's Club for $180-ish? Their 20-hour rate is a bit lower than I'd like at 190 AH but the price is right.

For the first two or three years, I will only be spending half the time on my land. Maintenance free solar specific AGMs would be nice but the Concorde SunExtenders and other solar industry specific AGMs are twice the price plus no one around here seems to stock them so freight would be more expensive. When not on my land, I'd either leave them to float or disconnect them and bring them home to put on tenders. I do have one local shop with some other GC2 batteries but he mostly deals with FLA and not many AGM. I'm not impressed with his Trojan T-105 pricing so his AGMs might be high as well. He does offer discounts on T-105's but only if you buy six and I don't really need six.

Once on my land full-time, I could then see myself spending on better quality batteries. In fact, by the time I am in my early 70's (I am 42 now), I'd probably buy a set of nice industrials that could last me 10 years and be my last set. I don't want to be swapping cells out in my later years. Actually, by my 60's, it would likely be time to swap out panels as well. Hopefully both the battery and panel market will have improved by then to the point I can buy my last set of both for the rest of my life.

Meanwhile, the Duracell GC2's are at a very attractive price point, with no shipping cost (I have no sales tax here in NH as well.), to start with especially with everything else I have to buy for my land.

Edit: Looks like the Duracell GC2's are made for Duracell by Deka/East Penn. That's a good manufacturer, no?

Edit2: The FLA Duracell GC2's are only $80 at Sam's but I'd definitely have to bring those back home with me to maintain them.

Blackcherry04

Re: Planning my 750W Off-Grid System - Am I doing it right?

The last time I checked on the AGM's at Sams they were like $139.00 + the core charge - $15. They will take any old battery in on the core trade in. Watch the age dating on them, I saw some over a year in a half old.

Tobit

Re: Planning my 750W Off-Grid System - Am I doing it right?

Blackcherry04 wrote: »

The last time I checked on the AGM's at Sams they were like $139.00 + the core charge - $15. They will take any old battery in on the core trade in. Watch the age dating on them, I saw some over a year in a half old.

They been good to you though Cherry? The website, based on my local Sam's, is quoting me $180 with no mention of a core charge. Good to know, I haven't been in.

Blackcherry04

Re: Planning my 750W Off-Grid System - Am I doing it right?

Tobit wrote: »

They been good to you though Cherry? The website, based on my local Sam's, is quoting me $180 with no mention of a core charge. Good to know, I haven't been in.

I'v never bought any AGM's there , all my customers have FLA either Duracell or Energizer's and I have over 300 of them, Managed or Owned, all of the banks are either 8 or 10 batteries. They are no problem, you get a year warranty and they honor it on the spot, without question. I have enough receipts nto take care of any problems. I think I had two batteries that I exchanged.

Tobit

Re: Planning my 750W Off-Grid System - Am I doing it right?

Blackcherry04 wrote: »

I'v never bought any AGM's there , all my customers have FLA either Duracell or Energizer's and I have over 300 of them, Managed or Owned, all of the banks are either 8 or 10 batteries. They are no problem, you get a year warranty and they honor it on the spot, without question. I have enough receipts nto take care of any problems. I think I had two batteries that I exchanged.

Should I just get the FLA's then for $80 ea. and bring them home with me for maintenance when I leave my land?

Blackcherry04

Re: Planning my 750W Off-Grid System - Am I doing it right?

Tobit wrote: »

Should I just get the FLA's then for $80 ea. and bring them home with me for maintenance when I leave my land?

You could, there about 70# and you need a strap to pick them up. How many are you going to have ? They are like any battery if you keep them on float. Someone here keeps theirs in their truck and just plugs them in with a cable when they are at the cabin. GC-2's are very easy to charge and keep balanced, call them predictable.

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Tobit

Re: Planning my 750W Off-Grid System - Am I doing it right?

Blackcherry04 wrote: »

You could, there about 70# and you need a strap to pick them up. How many are you going to have ?

Just 4 for a 24V bank.

Edit: I do see that the 20 AH rate is better on the FLA, 215 vs. 190

Blackcherry04

Re: Planning my 750W Off-Grid System - Am I doing it right?

Tobit wrote: »

Just 4 for a 24V bank.

Edit: I do see that the 20 AH rate is better on the FLA, 215 vs. 190

They are little more capacity. I wouldn't haul them, but I have 21 batteries, so it's out of the question. As long as you keep them healthy ( SG's up ) you don't need much charge capacity maybe 1 amp or so to keep them maintained, unless theft is a problem. There plenty of small battery maintainer solar panels out there, cheap.

Many build a battery box below ground to keep them in.

Tobit

Re: Planning my 750W Off-Grid System - Am I doing it right?

'coot, BB, et al., this is probably a dumb question.

If I am hooking up two CC's and an inverter to a single battery bank, I should probably connect the batteries to a bus bar first with the largest diameter wire I can use correct? I don't want three different wires bolted directly to a battery terminal, right? That could mess with torque and other things, eh?

Plowman

Re: Planning my 750W Off-Grid System - Am I doing it right?

Tobit wrote: »

'coot, BB, et al., this is probably a dumb question.

If I am hooking up two CC's and an inverter to a single battery bank, I should probably connect the batteries to a bus bar first with the largest diameter wire I can use correct? I don't want three different wires bolted directly to a battery terminal, right? That could mess with torque and other things, eh?

I just installed bus bars (Outback) on the batteries in my little off grid system. I previously had everything bolted directly to my batteries. Much cleaner install and easier to connect/disconnect different components without having to disconnect everything at once. Having everything connected directly to the battery is a PITA, and regularly connecting and disconnecting stuff when fiddling with the system can't be good for the terminals.

Having bus bars also allowed me to put a small circuit breaker on the cable leading to the positive bus bar (poor man's version of the Blue Sea terminal fuse)---I'm hoping this will protect the wires against a short circuit of the batteries.

I ran the biggest cable I had on hand (4 ga) from the batteries to the bus bars, which are only a few inches from the batteries. I only have two Trojan T-105s, a 45-amp charge controller (with only one 158W panel at the moment), and a 150W inverter. My understanding is that 4 ga is fine for this.

ETA: The Outback bus bars also allow you to easily run different sized wires to the batteries without having to deal with ring terminals. Not always easy to find/install the same sized ring terminals for wire ranging from 16 ga to 4 ga (the range of wires I currently have connected). Some bus bars (Blue Sea I think, and others) require ring terminals.